as I read ISO 639 extensions suggested, they don't seem to be
progressing towards sublanguages at all.
The reason I went from _ to - was this comment from Rhys:
I would prefer '-' rather than '_', mainly because I see future
possibilities for using these language tags in text/enriched
to get around some of the unification headaches of ISO-10646
(no, I'm not suggesting we do it now), and '-' fits in nicer
with the command syntax of text/enriched.
And this from Olle Jarnefors:
+ As separator between the two parts of a <langauge-token> "-"
is used instead of "_". This is of course a minor point, but
I think that this change is justified by making IETF language
codes easily distinguishable from the more limited language
codes of the form "en_US" used in Posix and X/Open locale
Also, I share a little of ISO's distrust about the _ character....
If ISO 639 does come up with a sublanguage registry, we will just
have to change our standard to reference the new one properly, I think.